Diffusion washing of comminuted cellulosic fibrous material has been practiced since the 1960s in large cylindrical vessels containing reciprocating screen assemblies. One prominent design is built and marketed by Ahistrom Machinery Inc. of Glens Falls, N.Y. under the name Atmospheric Diffusion Washer. These screen assemblies, referred to as "diffusers", typically comprise spaced concentric rings with perforated screen plates on the internal and external surfaces of the rings. Treatment liquids are typically distributed by rotating arms with upward or downward pointing distribution nozzles. Typical conventional designs are shown in U.S. Pat. Nos. 5,183,536 and 5,116,476.
This concentric ring design is not new, but was a characteristic of the very first diffuser designs. Examples of these early designs are shown n U.S. Pat. Nos. 3,348,390; 3,524,551; 3,563,891; and 3,760,948. Ostensibly, this circular ring design appears to be a preferred geometry for the diffusion of wash or bleaching liquids through a bed of medium consistency (i.e. about 8-15%) wood pulp. The circular rings provide not only an aesthetically pleasing, symmetric appearance but also appear to provide the optimum diffusion of treatment fluid through an upwardly flowing medium consistency bed of cellulosic material, e.g., wood pulp. The annular spaces between the rings form uniform pulp beds to which treatment medium can be applied and then extracted through the adjacent screens. The efficacy of this design has been confirmed by the hundreds of diffusers sold, and still being sold, since the 1970s. The Ahlstrom Atmospheric Diffuison washer is recognized today as one of the leading technologies in medium consistency pulp treatment.
However, regardless of the technological and commercial success of these devices there are some shortcomings of the concentric ring design that have shown this design to be less than optimum. For example, the concentric ring design produces a non-uniform resistance to the upward flow of pulp. The vertical screen plates of a typical diffuser design produce friction between the screen plates and the pulp flowing past it. This resistance to flow is directly proportional to the area of the screen over which the pulp flows. Since the area of screen to which each annular pulp bed is exposed increases as the diameter of each ring increases, the resistance to pulp flow is greater in the outer pulp annuli than in the inner annuli. In practice, this phenomenon is seen as a faster pulp flow in the inner annuli than in the outer annuli. This gradient in pulp velocity produces non-uniform treatment of pulp, or, in extreme cases, "channeling" of faster flowing pulp passed regions of slower moving pulp.
Another drawback of the present circular diffuser designs is the disruption of the pulp bed by the support structure of the rings. In all conventional diffuser designs the concentric rings are supported by some form of radial arms or beams which support the rings and provide part of the means for reciprocating the assembly. (Conventional diffusers are reciprocated to wipe the screen plates clean and prevent pluggage of the screen plates.) Several different non-circular configurations are illustrated in U.S. Pat. No. 4,276,167, but these designs all employ some form of radial or transverse support arms which traverse the pulp bed. However, these radial arms traverse the pulp flow path and disrupt the bed of pulp either entering or leaving the annulus between the screens. Not only do these arms interfere with the uniform flow of pulp, but the cracks and crevices produced also provide flow paths for treatment liquids. These flow paths can produce non-uniform concentration or "channeling", of treatment liquids in the pulp bed. The combined effect of the disruption of the pulp bed and channeling of the treatment medium produce inefficiencies that are manifest as non-uniform treatment of pulp and increased chemical consumption.
The rotating liquid distribution arms and nozzles also disrupt the pulp bed and promote non-uniform treatment. Elimination of these elements can not only improve treatment efficiency but also eliminate the drive mechanism and assorted hardware associated with these distribution arms.
The present invention avoids all of these shortcomings of the concentric ring design and provides a much more efficient treatment of the pulp.
A new approach, primarily for an atmospheric diffuser for washing or bleaching cellulose pulp is proposed, although some aspects of the invention may also be applicable to pressure diffusers (at a pressure of more than about 2 atmospheres). The objective is to get high efficiency with no moving parts (in-line), a modular design for improved expandability, and with ready accessibility for maintenance.
As described in U.S. Pat. No. 5,183,536 conventional atmospheric diffusion washer design includes an inherent inefficiency, namely, the dirty "backflush" liquid is undesirably re-introduced to the pulp bed. The actual data from tests performed on conventional atmospheric diffusion washers to determine the extent of this inefficiency appear in Table 1 below. Table 1 contains the average Norden efficiency numbers, often expressed as "E.sub.k " numbers, for several atmospheric diffuser operating conditions. The Norden efficiency number provides a relative indication of the washing efficiency of a pulp washing device or stage of a device. The higher the E.sub.k number the higher the washing efficiency of the device or stage. The efficiencies in Table 1 are based upon the removal of both the dissolved sodium and dissolved wood solids from the pulp.
TABLE 1 AVERAGE NORDEN NUMBER 1st 2nd (Sodium and Dissolved Solids) OVERALL STAGE STAGE With Backflush on Both Stages 7.7 .+-. 4.7 .+-. 3.0 .+-. 0..5 0.3 0.4 Without 1st Stage Backflush 8.5 .+-. 5.5 .+-. 3.1 .+-. 1.5 0.9 0.7 Without 1st and 2nd Stage Backflush 9.5 .+-. 6.4 .+-. 3.1 .+-. 1.4 1.4 0.2
In a conventional two-stage atmospheric diffusion washer, the stages comprise concentric annular screen assemblies mounted for reciprocation within a circular vessel. The pulp flow through these stages is vertically upward and the first stage is mounted beneath the second stage as shown in FIG. 1 of U.S. Pat. No. 5,203,045. During operation, wash water is introduced to the screen assemblies by rotating wash distribution nozzles and then extracted by the annular screens, for example, as clearly shown in FIG. 2 of U.S. Pat. No. 5,203,045. At the same time, the screen assembly is raised by hydraulic cylinders at the approximate speed of the upflowing pulp and then rapidly lowered, or "downstroked". Prior to downstroke, some of the flow of extracted liquid is momentarily reversed, or "backflushed", to dislodge the pulp bed from the screen prior to the downstroking. This very desirable, if not essential, displacement of the pulp bed from the screen assembly prior to downstroking releases the pulp bed from the screen such that the downstroking of the diffuser is neither hindered by the pulp bed, nor is the pulp bed disrupted by the stroking action of the diffuser. However, it is this backflushing of previously extracted liquid that is associated with reducing the washing efficiency of these devices. The data in Table 1 correspond to the overall efficiency and the individual efficiency of each stage of two-stage atmospheric diffusion washer.
The first line of data in Table 1 is associated with normal operation of the two-stage diffuser, that is, with backflushing during both the first and second stages, which is the baseline efficiency for this study. Though the overall efficiency for such a device corresponds to an average E.sub.k of 7.7, most of this efficiency is attributed to the first stage, E.sub.k of 4.7, and less to the second stage, E.sub.k of 3.0. The second line of data corresponds to operation of the two-stage device without backflushing in the first stage. As shown, the overall efficiency increases to 8.5; however, this increase largely occurs in the first stage which compared to the baseline data increased by 0.9 E.sub.k units. The second stage only increased only 0.1 E.sub.k units compared to the baseline.
The third line of data in Table 1 corresponds to operation of the two-stage device without backflush in either of the two stages. Again, the overall efficiency increased under these conditions, however, again, the majority of the increase was seen in the first stage and little or no increase in efficiency was measured for the second stage under these conditions. These results are generally interpreted to mean that backflushing in the first stage has a much more detrimental effect upon the efficiency of a two-stage diffuser. In particular, as described U.S. Pat. No. 5,183,536, this inefficiency is attributed to the location of point of introduction of the backflush liquid relative to the clean pulp. In the lower stage diffuser assembly, the dirty backflush liquor is introduced to a cleaner flow of pulp. This is not the case in the upper stage of a two stage diffuser assembly, as indicated by the data in Table 1. As such, it is undesirable to backflush dirtier liquid into a pulp stream and elimination or minimization of such backflushing is desirable. The present invention provides a diffuser-type washing device which preferably does not require such backflushing.
A second source of inefficiency associated with conventional atmospheric diffusers is the disruption of the pulp bed by the support arms and the consequent source of channeling that this disruption promotes. As the pulp bed rises through the annular screen assemblies it is surmised that the radial support arms provide obstructions to the uniform flow of the annular pulp bed. This disruption and the cracks that are produced undesirably introduce flow paths for the treatment liquid, or channeling of the treatment liquid, that interfere with treatment efficiency. This bed disruption has also been recognized as another source of the inefficiency in the second, upper stage of a conventional two-stage diffuser compared to the first, lower stage. The lower stage treatment zones are not disrupted by the typical radial support structure prior to or during treatment, while the upper stage is located directly above such radial support structures.
In addition, the method of distributing the treatment liquid by rotating nozzles, as shown in FIG. 2 of U.S. Pat. No. 5,203,045, and which characterizes the prior art atmospheric diffusion washers, also introduces undesirable disruptions of the pulp bed which introduces flow paths that promote treatment liquid channeling. It is desirable to minimize or eliminate any source of disruption of the pulp bed, for example, by using a support structure or liquid distribution nozzle that does not traverse the flow path prior to or during treatment. The present invention is also characterized by the minimization or elimination of such structures to provide an undisturbed bed of material for treatment.
The present invention also addresses the washing inefficiencies that are due to the variation in pulp bed permeability that are characteristic of prior art diffusion washing devices. A bed of comminuted cellulosic fibrous material, for example, wood pulp, is compressible under the force of a flow of liquid through the bed. As liquid flows through a bed of pulp, the viscous drag force of the liquid on the pulp compresses the pulp bed in the direction of flow. As the bed is compressed and the interstitial spaces between the pulp particles are reduced in size, a greater restriction to flow is produced. This compression and restriction of flow increase with the flow velocity of the liquid passing through the pulp bed. Ultimately as the flow increases, the compression restricts flow so that little or no flow can pass through the pulp, that is, the permeability of the pulp bed decreases, approaching zero permeability, or no flow. This flow restriction is also a function of the consistency of the pulp and the viscosity of the fluid, among other things. For example, the higher the consistency, the greater the flow restriction.
This permeability phenomenon has a recognized impact upon the operation of diffuison-type washers. Typical values of permeability for the flow from a wash distribution nozzle to a screen are shown in FIG. 15. FIG. 15 illustrates the permeability of the pulp bed between the nozzle and the screen of a conventional diffusion washer for various liquid flow velocities, v. Correspondingly, as the liquid velocity through the bed increases, the permeability decreases and the pressure drop across the pulp bed increases. The typical relationship between liquid velocity and pressure drop across the pulp bed is shown in FIG. 16.
In addition to varying across a pulp bed, the permeability of a bed of pulp can vary along the length of the screen which extracts liquid from the bed. For example, when a bed of pulp encounters an extraction screen, the initial removal of liquid by the screen creates a compression of the pulp bed nearest the screen. Again, this compression of the pulp bed locally decreases the permeability of the pulp near the screen. As the bed of pulp continues to move along the screen, the compression of the pulp bed continues to increase with a consequent loss in permeability. Thus, along the height of an extraction screen in a diffusion-type washer, the permeability may vary dramatically. Tests in a pilot scale diffusion washing device have shown that the permeability may decrease along the height of an annular screen plate so that liquid passes through only part the lower section of screen. That is, in the upper section to the screen, the effect of compressibility and loss of permeability of the pulp bed can prevent the passage of liquid through the bed and to the screen. The efficiency of the conventional device may also suffer from this variation in permeability along the length of the screen. However, in one of the preferred embodiments of this invention, this variation in permeability is recognized and accounted for such that diffuser-type washing efficiency can be markedly enhanced.
Another aspect of any type of treatment of a bed of pulp with a device using a screen-type barrier for isolating liquid from the pulp, for example, for washing, de-watering, or bleaching, is that some accommodation must be made to prevent plugging of the screen surface. In conventional diffusion washers, this pluggage is minimized through backflushing and downstroking. Other prior art, for example, as disclosed in U.S. Pat. No. 4,076,623, suggest vibrating or oscillating the screening surface to dislodge the pulp and minimize screen pluggage. In ether case, some form of relative movement between the screen surface and the pulp bed is used to minimize screen pluggage and maintain operation of the device.
However, it is undesirable that such movement disrupt or somehow disturb the bed of pulp during treatment. Diffusion washing is preferably performed with a uniform flow, or diffusion of treatment liquid, through a uniform pulp bed with little or no disruption of the pulp bed. Disruption or disturbing the pulp bed can lead to non-uniform flow of liquid through the bed or non-uniform flow of pulp through the treatment zone which can lead to non-uniform or less than desirable treatment of the pulp. This non-uniform flow of either liquid or pulp is typically referred to as "channeling" of the liquid or pulp and is undesirable. It is preferred that any relative movement of the screen or other device not interrupt or disturb the pulp bed.
The potential for disturbing the pulp bed is highly contingent upon the strength of the pulp bed, that is, how much load or pressure the pulp bed can withstand without "breaking" or shearing or otherwise promoting inefficient pulp or liquid channeling or mixing. A typical relationship between pulp bed strength and consistency (also known as "Solids Fraction") is shown in FIG. 17. As the consistency of a bed of pulp increases, the load which the bed can withstand without being disrupted increases. The present invention seeks to limit the disruption of the pulp bed exceeding the strength of the bed.
The present invention utilizes the solids flow design principles and pulp bed compaction and permeability principles used in the development and design of Diamondback.RTM. chip bins, or like vessels, developed and marketed by Ahlstrom Machinery Inc. of Glens Falls, N.Y. (such as shown in U.S. Pat. Nos. 5,500,083 and 5,617,975). In addition these principles are also employed to provide smooth flow transitions from the treatment zone of the treatment vessel to the discharge of the vessel.
While in the description of the present invention provided, the invention is described with respect to washing of a liquid slurry of solid material, particularly medium consistency pulp, it should be understood that the invention also is applicable to simply thickening the pulp (that is removing liquid from it but not adding additional displacement liquid), and for other treatments aside from washing, such as using a bleaching liquid to bleach the pulp, etc. Though this invention can be practiced in a pressurized vessel, the invention is most suitable for substantially atmospheric pressure treatment. Normally the treatment will be at a pressure slightly above atmospheric, but in a preferred embodiment no attempt is made to specifically pressurize the vessel in which treatment takes place, and the pressure is always significantly less than two atmospheres.
According to one aspect of the present invention a diffusion assembly is provided for treating a liquid slurry of solid material (e.g. cellulose pulp at a consistency of about 8-15%). The assembly comprises the following components: A substantially upright vessel (e.g. at substantially atmospheric pressure) have a non-circular cross-section, defined by a vessel wall, for at least a treatment portion thereof, the treatment portion between an inlet: and an outlet for liquid slurry. At least one screen surface for removing some of the liquid of the liquid slurry from the treatment portion of the vessel, but minimal solids slurried by the liquid. And, the vessel and the at least one screen surface, constructed and positioned with respect to each other so that during treatment the liquid slurry has a substantially uniform width in the treatment portion and there is substantially uniform resistance to the flow of slurry over the screen surface over any particular cross-section of the vessel in the treatment portion.
While the vessel may have a substantially square, rectangular, oval, or like configuration, in the preferred embodiment the vessel has a substantially race track shape in cross-section at the treatment portion. The race track shape minimizes the size, or area, of the device. For some aspects of the invention the configuration may be circular.
In one embodiment the screen surface is mounted interiorly of the vessel treatment portion, and the assembly further comprises treatment liquid introducing means (such as slots, headers, apertures, baffles, nozzles, orifices, and/or other conventional liquid introducing structures) operatively mounted in or with the vessel wall for introducing treatment liquid into the slurry in the vessel treatment portion.
Typically the screen surface is substantially vertical and comprises screening portions vertically spaced by non-screening portions. In order to provide optimum permeability for the pulp as it moves through the vessel, the vessel wall bulges out substantially at the location of the non-screening portion so as to provide a slurry width thereat at least five percent wider (e.g. 5-30% wider, and all smaller ranges within that broad range) than at the screening portions associated therewith, so as to allow the pulp to "relax" to regain at least some of the permeability that it had before passing into contact with the screening portions. This recovery of permeability permits the further treatment of the pulp.
The screen surface may be mounted interiorly of the vessel by a plurality of hollow supports which both support the screen surface and remove liquid which passes through the support surface from the vessel, and the assembly may further comprise means for moving the screen surface at spaced points in time so as to substantially prevent plugging of the screen surface. The moving means may be conventional reciprocating devices (such as pneumatic cylinders), vibrators which vibrate the screen surface up and down and/or horizontal, or any other conventional such structure. Alternatively the means may be for rotating the screen surface about a substantially vertical axis, or for vibrating or oscillating the screen surface in a substantially horizontal dimension.
Typically the vessel inlet is at the bottom thereof and the vessel outlet is at the top. A one dimensional convergence and side relief structure may be directly connected to, or form at least part of the outlet.
The vessel, liquid introducing means, and at least one screen surface may be constructed and positioned with respect to each other so that the maximum and minimum permeability of a bed of the liquid slurry during passage through substantially the entire treatment portion of the vessel is a maximum of about 1000 lbs. per square foot per foot, preferably a maximum of 750 lbs./ft..sup.2 /ft., most preferably a maximum of 500 lb./ft..sup.2 /ft. That is the consistency is never more than 18%, preferably less than 16%, most preferably less than 14%, and typically is between about 8-14%. The slurry width may form an annulus in the vessel, and the annulus is preferably substantially uninterrupted in the treatment zone, there being no significant physical impediment such as arms in conventional diffusers. The assembly is typically provided in combination with a plurality of like vessels, and an exterior vessel, the exterior vessel mounting the plurality of like vessels within it. While the vessels are like each in the configuration, they may differ in dimension within the exterior vessel.
The invention also relates to a method of treating a slurry of compressible comminuted cellulosic fibrous material (e.g. pulp having a consistency of between about 8-15%), having a permeability that varies as it is compressed, in a treatment zone. The method comprises: (a) Introducing a flow of the slurry into the treatment zone (e.g. which may be at substantially atmospheric pressure) to form a bed of slurry having a first side and a second side, and a width. (b) Passing the slurry through the treatment zone. (c) In the treatment zone introducing a treatment liquid into the bed at the first side thereof so that the liquid passes from the first side through to the second side thereof. (d) Removing at least some of the liquid from the second side of the bed. And, wherein (a)-(d) are practiced so that the permeability of the slurry bed in the treatment zone is a maximum of 750 lbs./ft..sup.2 /ft. (e.g. a maximum of 500 lbs./ft..sup.2 /ft.).
In the method (b) is preferably practiced substantially without disrupting the flow of slurry (that is with no arms or like physical impediments as in conventional diffusers). In the method (c) and (d) are preferably not practiced at spaced locations as the slurry moves through the treatment zone, and the method further comprises increasing the width of the slurry bed at the spaced locations where (c) and (d) are not practiced, to allow the slurry to recover substantially its permeability before practice of (c), e.g. by increasing the width at least about five percent. In the method (a) and (c) are typically practiced in substantially vertical paths, whereas (c) is practiced in a substantially horizontal path.
In the method (a)-(d) may be practiced to establish a plurality of substantially parallel beds of slurry in treatment zones, and to treat the slurry in all of the plurality of beds and treatment zones at substantially the same time. The method also comprises combining the plurality of beds after treatment thereof, such as for further treatment (e.g. in a bleach plant or the like). In the method (c) is typically practiced using the wash liquid to effect washing of the material of the slurry, but other treatment processes may also be carried out, such as bleaching or delignification.
According to another aspect of the present invention there is provided a method of treating a slurry of compressible comminuted cellulosic fibrous material as described above in a treatment zone comprising: (a) Introducing a flow of the slurry having a consistency of between about 8-15% into the treatment zone to form a bed of slurry having a first side and a second side, and a width. (b) Passing the slurry through the substantially atmospheric pressure treatment zone. (c) In the treatment zone introducing a treatment liquid into the bed at the first side thereof so that the liquid passes from the first side through to the second side thereof. (d) Removing at least some of the liquid from the second side of the bed. And, wherein (a)-(d) are practiced so that the slurry has a consistency at all times throughout the treatment zone of between about 8-18% (preferably 8-16%, most preferably 8-14%; and a difference of less than 6%); and wherein (b) is practiced substantially without disrupting the flow of slurry.
The details of the individual elements of the method practiced may be as described with respect to the previous aspect of the invention. In either aspect, (d) may be practiced by passing the slurry into contact with a screen surface, liquid passing through the screen surface. The method may further comprise at spaced points in time moving the screen surface to substantially reduce surface friction, but preferably by rotating the screen surface about a substantially vertical axis, or vibrating or oscillating the screen in a horizontal direction (substantially transverse to the direction of movement of pulp through the vessel)).
The present invention may consist of or comprise a method of treating a compressible comminuted cellulosic fibrous material, having a permeability that varies as the material is compressed, in a treatment zone, including: (a) introducing a flow of comminuted cellulosic fibrous material to a treatment zone to form a bed of material having a first side and a second side and a width; (b) passing the material through the treatment zone; (c) introducing a treatment liquid to the first side of the bed; and passing the treatment fluid from the first side of the bed to the second side; and (d) removing at least some of the liquid from the second side of the bed.
The method is preferably practiced so as to substantially prevent the loss of permeability of the bed to the treatment fluid. In a preferred embodiment, (b) is practiced substantially without disrupting the bed of material in the treatment zone. In another embodiment, (c) and (d) are practiced by intermittently stopping the removal of treatment liquid from the material. In another embodiment, while performing (c)-(d), the method also comprises allowing the material to recover at least some of the permeability lost during the practice of (c)-(d). One preferred way of recovering at least some of the lost permeability is by increasing the width of the bed.
According to another aspect of the invention there is provided a method of treating a slurry of compressible comminuted cellulosic fibrous material, having a permeability that varies as it is compressed, in a substantially annular [e.g. substantially atmospheric pressure] treatment zone, comprising: (a) introducing a flow of the slurry having a consistency of between about 8-15% into the treatment zone to form a bed of slurry having a first side and a second side, and a width; (b) passing the slurry through the [e. g. substantially atmospheric pressure] treatment zone; (c) in the treatment zone introducing a treatment liquid into the bed at the first side thereof so that the liquid passes from the first side through to the second side thereof; (d) removing at least some of the liquid from the second side of the bed; and practicing (a)-(d) so as to positively control in the treatment zone the pressure drop across the bed, the friction load, the consistency of the slurry, the inverse of permeability, and the inverse of bed strength, so as to limit the negative impact thereof upon treatment efficiency.
The invention also relates to a diffusion assembly for treating a liquid slurry of solid material, the assembly comprising: A substantially upright vessel defined by a vessel wall, for at least a treatment portion thereof, the treatment portion between an inlet and an outlet for liquid slurry. At least one screen surface for removing some of the liquid of the liquid slurry from the treatment portion of the vessel, but minimal solids slurried by the liquid, the screen surface substantially vertically elongated. And, means for occasionally moving the screen surface in a direction or manner distinct from the vertical so as to minimize plugging thereof.
The moving means may comprise means for rotating the screen surface about a substantially vertical axis less than 30 degrees (e.g. 5-30.degree.) or means for vibrating or oscillating the screen surface in a substantially horizontal direction.
The invention also relates to a diffusion assembly for treating a liquid slurry of solid material, the assembly comprising: A substantially upright vessel defined by a vessel wall, for at least a treatment portion thereof, said treatment portion between an inlet and an outlet for liquid slurry. At least one screen surface for removing some of the liquid of the liquid slurry from the treatment portion of the vessel, but minimal solids slurried by the liquid, the screen surface substantially vertically elongated and comprising screening portions vertically spaced by non-screening portions. Treatment liquid introducing means for introducing treatment liquid into the slurry in the vessel treatment portion. A volume through which the slurry flows between the screen surface and the treatment liquid introducing means in the treatment portion, the volume having a width. And, the vessel, screen surface, and liquid introducing means positioned with respect to each other so that the width in the treatment portion increases by at least about 5% at at least one of the non-screening portions.
The treatment liquid introducing means may be mounted in or operatively associated with the vessel wall, and the screen surface is mounted interiorly of the vessel, the vessel wall may bulge out substantially at the location of the non-screening portions so as to provide the wider slurry width thereat. The means described above for occasionally moving the screen surface in a direction or manner distinct from the vertical may also be used.
It is the primary object of the present invention to provide a method and apparatus which substantially overcomes the washing inefficiencies due to the variation in pulp bed permeability that are characteristic of conventional diffusion washers particularly atmospheric diffusion washers. The stationary operation of the present invention also provides for a less expensive and easier to maintain device. The modular design also allows for easier and less costly capacity increases. This and other objects will become clear from an inspection of the detailed description of the invention and from the appended claims.